Titanium-Sulfur-Based Amorphous Metals

Stronger Than Steel, Elastic and Formable Like Plastic

Fig. 1: Amorphous metals combine the high strength (yield point) of metals with the elasticity of plastics (after Ref. [3]).

Fig. 2: Schematic time-temperature-transformation (TTT) diagram. In order to obtain an amorphous metal, the melt is cooled so fast during the casting process that crystallization cannot occur. Avoiding crystallization, the melt solidifies in its random structure and forms a glass. Amorphous products, e.g. powder, can then be deformed or consolidated at rather low temperatures above the glass transition Tg.

Fig. 3: Specific strength and hardness of the newly developed sulfur-bearing amorphous metals. The values for the crystalline titanium alloy Ti6Al4V and the amorphous alloy AMZ4 are given as a reference. E indicates the elastic modulus of the alloys. Data from Ref. [6].

Amorphous metals, or metallic glasses, experienced a rapid development during the last 50 years. They now found their way out of the laboratory and are a promising candidate for industrial application. The recent development of titanium-sulfur-based amorphous metals might be the final step towards an economic application of this material class.

The Benefit of Atomic Chaos

At first view, amorphous metals and conventional metals look alike. Only on the atomic level it is visible that the atoms are not arranged in a periodic crystal lattice, they are rather randomly distributed. And exactly this atomic chaos causes the special properties of amorphous metals. On the one hand they display an enormous elasticity of 2 %, and on the other hand, they possess an extraordinary strength (yield point) (fig. 1) right after casting that does not require any further heat treatment. Often, amorphous metals were related to a rather brittle fracture behavior, however, research in recent years has shown that compositions with excellent ductility exist [1]. There are amorphous metals that can be heavily deformed, even at room temperature [2].

Production, Development and Processing

Amorphous metals are typically complex alloys with low melting- and liquidus temperature. The alloys can be processed by tilt-casting or by pressure-supported casting methods. The production process is visualized in the time-temperature-transformation (TTT) diagram in figure 2. In order to preserve the amorphous structure of the melt during the casting process, the onset of crystallization has to be avoided. The time- and temperature dependent crystallization process is schematically shown as the typical crystallization “nose” in figure 2. The development of new amorphous alloys aims at shifting the onset of crystallization to longer times. Nevertheless, the production of amorphous parts still requires casting the melt into molds with high heat conductivity in order to obtain a high cooling rate.

The time to reach the onset of crystallization defines the so-called “critical cooling rate”, down to which the amorphous structure of the melt can be preserved during the casting process.

The critical cooling rate is also linked to the maximum wall thickness of parts that can be produced fully amorphous (critical casting thickness). In all cases, the limiting factor is the removal of heat from the center of the casting part. Amorphous metals are considered as “bulk” (bulk metallic glasses, BMG) if they reach a critical casting thickness of at least 1 mm. This translates into a time of ~4 × 10-3 s until the tip of the crystallization nose. In certain alloy systems a critical casting thickness of up to 80 mm (tip of the crystallization nose at ~2,8 × 102 s) can be reached [4].

When successfully bypassing the crystallization nose during casting, the viscosity of the melt rapidly increases upon further cooling, until the melt freezes at the glass transition temperature Tg and forms a solid (glass). In contrast to conventional alloys, the transition from liquid to solid does not display a sudden decrease in volume, thus preventing void formation due to shrinkage. Therefore, surface structures on the µm-range can be cast without additional machining and net-shape forming becomes standard.

The process of thermoplastic forming (TPF) offers the possibility to reshape amorphous parts or even to bypass the limitations given by the critical casting thickness. The TPF process resembles the forming process during the glassblowing of oxide glasses and is unique for metallic alloys. By heating the amorphous metal above its respective glass transition temperature Tg (100 - 500 °C, depending on the alloy) into the highly viscous range of the supercooled liquid (Fig. 2 - TPF), it can be reshaped, the surface can be structured, or amorphous powder can be consolidated into bulk material [5]. Furthermore, amorphous powder can be processed by additive manufacturing techniques (3D printing), where the position of the crystallization nose again plays an important role.

The Crucial Ingredient: Sulfur

Sulfur typically deteriorates the mechanical properties of conventional crystalline alloys and is therefore rarely a desired constituent. However, sulfur is widely used in the vulcanization process in the rubber industry. The fact that sulfur is easily available, even in high purity, makes it convenient for an industrial application. Just recently it was discovered that sulfur can also be a crucial ingredient in amorphous alloys, allowing glass formation in many new alloy systems. Within these alloy systems, especially the titanium-based alloys might pave the way for the widespread commercial use of amorphous metals in industry. Amorphous titanium alloys display an extraordinary strength and a high corrosion resistance, together with a low density, thus enabling applications in areas where other materials fail or bring in too much weight. Additionally, amorphous titanium alloys come with the previously mentioned advantages in terms of processability.

Before the discovery of sulfur as a constituent, amorphous titanium alloys with a titanium-content over 55 atomic percent (at%) – and hence a low density – always included the highly toxic element beryllium [6]. Now, an amorphous titanium alloy with a titanium-content of 70 at% (Ti70Zr5Ni12Cu5S8 [6]) was developed at the chair of metallic materials of Prof. Dr. Ralf Busch at Saarland University, in cooperation with the German Heraeus technology group. This alloy is based on the ternary Ti-Ni-S system, in which only in a very narrow compositional range of few at% the formation of a glass is possible. By partially substituting titanium with zirconium and nickel with copper, the range of glass-forming compositions was extended significantly, and the critical casting thickness was increased up to 4 mm. The low density of these alloys results in an extraordinary specific strength (0.57 GPa g-1 cm3 for Ti60Zr15Ni12Cu5S8) that by far exceeds the ones of crystalline titanium alloys, e.g. Ti6Al4V (0.35 GPa g-1 cm3), (Fig. 3). At the same time, the amorphous alloys display a hardness value in their as-cast state that is around 200 HV5 higher than the ones of crystalline alloys, while their elastic modulus E remains rather unchanged (Fig. 3).

With nickel in the alloys being undesirable for applications within or in contact with the human body, the research team at Saarland University has already developed Ni-free alloys (Ti60Zr15Cu17S8) and is further improving them.

Furthermore, sulfur was found to stabilize the supercooled liquid region of known alloys upon heating from the glassy state [6]. Two of these alloys are the already commercially used alloys Vit105 (Zr52.5Cu17.9Ni14.6Al10Ti5) and Vit101 (Cu47Ti34Zr11Ni8). These improvements are crucial for the processing of these alloys by thermoplastic forming and additive manufacturing processes.

The combination of outstanding properties, a high titanium content, and alloying elements suitable for industrial processing, for the first time allows amorphous titanium alloys to compete with and possibly win against their crystalline counterparts.